For oilfield and hydrocarbon exploration, a downhole drilling tool with a bit at an end thereof is advanced into the ground to form a wellbore. As the drilling tool is advanced, a drilling mud is pumped through the drilling tool and out the drill bit to cool the drilling tool and carry away cuttings. The drilling mud exits the drill bit and flows back up to the surface for recirculation through the tool. The drilling mud may also be used to form a mud cake to line the wellbore.
During and after the drilling operation, it is particularly useful to perform various downhole evaluations, such as testing, sampling, and/or scanning a subsurface geological formation to provide data representing the various strata and hydrocarbons that constitute the subsurface formation (referred to collectively herein as “logging”). In some cases, the drilling tool may be provided with devices to log the surrounding formation. In some cases, the drilling tool may be removed and a wireline logging tool may be deployed into the wellbore to log the formation. Logging may also be performed during other wellbore operations, such as treatment, production, etc.
The wellbore formed by the drilling tool is typically non-uniform and may contain obstructions, bumps or other non-uniformities in the wall of the wellbore. These non-uniformities (as well as other conditions in the wellbore) may cause damage to the downhole tool and/or its components.
Attempts have been made to provide downhole tools with devices that retract during transport through the wellbore and expand for contact with the wellbore wall. Some techniques involve downhole tools with extendable arms as described, for example, U.S. Pat. Nos. 4,614,250, 4,926,937, 4,979,585, 5,092,056, 6,702,010, 5,022,484, 7,069,775, 7,131,210. Various downhole tools with sensors positioned on extendable arms have been developed, such as the FSI™, NCMI™, and HDT™ tools, commercially available from SCHLUMBERGER™; the OMRI™ and EMI™ tools, commercially available from HALLIBURTON™; the STAR™ and EARTHIMAGER™, commercially available from BAKER HUGHES™ INC.; and the MICROIMAGER™, commercially available from WEATHERFORD™ INTL. Some attempts involve the use of logging tools for logging as they are passed through the wellbore, sometimes referred to as “log-down kits.”
Despite the development of techniques involving downhole tools with extendable arms, there remains a need to provide advanced techniques for effectively positioning sensors against the wellbore wall; and in particular, sensors used to provide an image of the formation's properties to a user/operator at the surface. It may be desirable to provide techniques that enable logging as the downhole tool passes through the wellbore. It may be further desirable to provide techniques to enhance contact with the wellbore wall. Preferably, such techniques involve one or more of the following, among others: bi-directional logging of the wellbore, increased contact of the pad with the wellbore wall, increased coverage of the wellbore wall, verification of the previously logged areas, enhanced (e.g., uniform) engagement with the wellbore wall, and/or resistance to sticking in the wellbore.